1. Field of the Invention
The present invention relates to processes and apparatus for the continuous production of slush hydrogen, and more particularly to a ground-based Slush Maintenance Unit (SMU) having a size and power capability enabling its incorporation into mobile ground service equipment adapted for continuously maintaining the fuel density and quantity required prior to takeoff of an aerospace vehicle which uses slush hydrogen (SH2) as a fuel.
2. Background of the Invention
One direction of aerospace research today is the development of long range, hypersonic space vehicles which will journey from earth to various points in outer space. Recently, vehicles of this type have originated from work under the National AeroSpace Program, commonly referred to as the NASP program.
For a NASP-type vehicle using slush hydrogen as a fuel, there is a requirement to maintain the slush quality (density) in the vehicle's tanks until a very short time before take-off to assure the correct propellant load for flight. Often unavoidably heat leaks into the propellant to cause a proportional amount of solid hydrogen to melt. To prevent a decrease in propellant density and a corresponding increase in propellant volume, with a potential overflow of liquid hydrogen, the melted hydrogen (triple-point liquid) must be continuously removed from the vehicle and replaced by an equal mass of solid hydrogen. Inasmuch as solid hydrogen alone cannot be transported through a pipe,,the hydrogen is transported as a slurry of triple-point liquid hydrogen and solid hydrogen (a mixture known as "slush hydrogen").
For a fixed "launch pad" vehicle, such as the Shuttle or the Saturn rocket, the slush hydrogen could be supplied from a facility storage tank, and the off-loaded triple-point liquid hydrogen could be collected and stored in a second tank.
On the other hand, "aircraft" vehicles such as the NASP-type vehicles must be fueled at a loading facility located remotely from the runway, and then the vehicle must be towed to the end of the runway for takeoff. Due to unavoidable heat leaks into the propellant during the tow period (up to 1 hour in duration), the melting solids in the SH2 will cause an expansion of the propellant volume and result in a potentially catastrophic spilling of liquid hydrogen from the tank vent valves. To prevent such spilling of liquid hydrogen, a mobile Slush Maintenance Unit (SMU) is required. The SMU will accompany the vehicle during the tow period as well as any subsequent hold periods prior to takeoff. The function of the SMU is to take the excess triple point liquid hydrogen from the vehicle, convert it back to slush hydrogen, and then return the slush hydrogen to the vehicle tanks to maintain the required fuel density and volume.
The essential components of a slush maintenance unit would be (1) a tank to catch the triple point liquid hydrogen returning from the vehicle, (2) a slush generator to convert the triple point liquid hydrogen back to slush hydrogen with an appropriate solid fraction, (3) a makeup tank of liquid hydrogen to replace the liquid hydrogen lost in the slush hydrogen production process, (4) a means to safely dispose of hydrogen vapor that is generated in the production process, and (5) an energy source to provide power for required compressors, pumps, heaters, etc.
The slush generator is the central component inasmuch as the operating characteristics and efficiency of the slush generator defines the power requirements, quantity of required makeup hydrogen, required capacity of the slush hydrogen disposal system, and the overall size of the combined unit.
The current, most common method of producing slush hydrogen is the "freeze-thaw" process. In this process, refrigeration for solidification of part of the hydrogen is obtained by cyclically reducing the ullage pressure in a container of liquid hydrogen to the triple point pressure of 1.02 PSIA. Evaporation of part of the liquid chills the bulk down to the triple point temperature of 24.8.degree. R. and the subsequent conversion of part of the bulk to solid hydrogen. The resulting mixture of triple point liquid hydrogen and solid hydrogen is "slush hydrogen".
To obtain the requisite low pressure over the bulk hydrogen, large capacity, low speed positive displacement vacuum pumps must be used. Typically, these vacuum pumps are driven by electric motors, which require large quantities of electrical power for operation. Thus, for slush hydrogen producing facilities of known capacities, a major power supply capability is required.
Due to the slow speed of the positive displacement vacuum pumps, the size and weight of the machinery becomes quite large for even a moderate refrigeration requirement. The problem is further aggravated by the fact that currently-available piston-type vacuum pumps require essentially room-temperature gas 60.degree. F., 520.degree. R.) at their inlets because of mechanical constraints.
An additional substantial amount of electrical power is required to heat the vapor from 24.8.degree. R. to 520.degree. R., thereby aggravating the facility electrical power requirement. Heating the vapor results in a volume increase by a factor of approximately 22, thereby greatly aggravating the size, weight, and power requirements for the vacuum pumps. In current practice, the hydrogen vapor discharged from the vacuum pumps is ducted to a burn stack for safe disposal. To dispose of the amount of hydrogen vapor to support a NASP-size vehicle will require a large burn stack located a substantial distance away from the vehicle, plus a long, large-diameter line to duct the hydrogen to the burn stack.
This situation, considering machinery size and weight, power requirements, and associated hardware for a slush maintenance unit based on current practice, would result in a formidable piece of Ground Service Equipment (GSE), even for a stationary, launch-pad-type of support facility. Incorporating such a slush management unit of this type into a mobile unit would require a piece of GSE approaching, or even exceeding, the size of the mobile launch platforms used for Saturn and shuttle launches. Moreover, supplying power to this mobile unit would require a very large on-board power generator, such as a diesel-electric generator. Moreover, the burn stack for disposal of the waste hydrogen vapor would resemble a large smoke-stack. The burn stack would have to be towed behind the mobile unit to provide continuous disposal of hydrogen vapor to prevent explosive accumulations of hydrogen.